Undifferentiated pleomorphic sarcoma (UPS), formerly referred to as malignant fibrous histiocytoma, is a type of soft tissue cancer. The word "undifferentiated" in undifferentiated pleomorphic sarcoma means that the cells don't resemble the body tissues in which they develop. The cancer is called pleomorphic (plee-o-MOR-fik) because the cells grow in multiple shapes and sizes.

While sarcomas are not common tumors, they do represent one of the most common soft tissue malignancies in adults. Soft tissue sarcomas can develop in blood vessels and in deep skin, fat, muscle, fibrous or nerve tissues.

In particular, UPS usually occurs in the arms or legs, and somewhat less often in the area behind the abdominal cavity (retroperitoneum). The cancer typically becomes quite large over a period of weeks or months, sometimes growing quite rapidly. In most cases, only a rapidly growing UPS causes noticeable pain. Although it tends to grow locally, the cancer can spread to other locations in the body, most often the lungs.

Undifferentiated pleomorphic sarcoma is called a diagnosis of exclusion, meaning it's agreed upon when other diagnoses have been ruled out; it represents less than 5 percent of adult sarcomas.


  • The most common sign of undifferentiated pleomorphic sarcoma is an enlarging, (usually) painless mass, which can become quite large, especially if it develops in a thigh or the retroperitoneum.
  • Sarcomas in the retroperitoneum can also cause pain, lack of appetite, constipation, or bowel or stomach bleeding. The mass can grow big enough to be felt in the abdomen.
  • In cases of extremely rapid growth, you may also feel pain or a sense of compression around the growth, or you may have unexplained tingling, pricking or burning sensations on your skin, or fluid buildup in an affected foot or other extremity.
  • More rarely, you may develop systemic signs and symptoms such as fever or weight loss.

When to see a doctor

Make an appointment with a doctor if you develop any signs or symptoms that seem unusual or that worry you:

  • A new lump or a lump growing on your body
  • Abdominal pain that is getting worse
  • Vomiting
  • Bloody stools
  • Black, tarry stools (when bleeding happens in the stomach or bowels, the blood can turn black as it is digested, and may make stools very black and sticky)

Most often these signs and symptoms come from some other issue than sarcoma, but consult with your doctor regardless.


Although the causes of UPS are largely unknown, a small percentage (less than 2 to 3 percent) develops at the site of previous radiation therapy. In very rare cases, the sarcoma can also develop in or around chronic ulcers or tissue scarring. A specific cause isn't often identified for most UPS.

Risk factors

Most undifferentiated pleomorphic sarcomas develop in people with no known risk factors. However, some factors may increase their likelihood, including radiation, a damaged lymph system and exposure to certain chemicals.

Again, most people who develop undifferentiated pleomorphic sarcoma have no known risk factors, and many people who have risk factors never develop the cancer.